A pawn storm approaches and it's time for the king to flee.
20. c3c4  20... g8h8 This is probably not the best square for the king. 21. d1f2  21... g4g2 Threatening capture on e4 followed by ...Rxf2, when the rook on h1 is left undefended. 22. e1e2 The only move to defend against Black's threat.